1. Field of Invention
This invention relates to helicopter tail rotors and more particularly to helicopter tail rotors which are of the flexrotor or composite bearingless rotor design in which a torque tube, which forms the blade inner end, envelops the torsionally flexible structural spar, and including a control wheel member spaced from and connected to the torque tube inner end by push-pull rods so that as the control wheel member is caused to rotate relative to the torque tube inner end, the push-pull rods will both position the torque tube relative to the flexible spar member and cause the spar member to twist, thereby causing blade pitch variation.
2. Description of the Prior Art
In the early flexrotor or composite bearingless rotor designs, a torque tube was used in a fashion in which the torque tube was cantilevered from the outer end of the flexible beam spar and extended inwardly therefrom to form the blade inner end which was connected to a pitch control push rod. This early design was extremely attractive from the standpoint of simplicity, low weight and having a minimum of parts, however, it proved to have undesirable pitch-flap coupling characteristics and was potentially unstable. A snubber or support member, for example of the type taught in U.S. Pat. No. 4,087,203, was therefore added between the flexbeam spar and the torque tube inner end near the pitch control push rod attachment point so that the push rod load could be reacted without coupling with pitch. This design also incorporated a flatwise stiffness at the interface of the flexbeam spar and the torque tube so that the large bending moments would not be carried across this region and thereby avoid the creation of high stresses. This design is used in the Army BLACK HAWK helicopter and was found to be completely acceptable from the standpoint of stability and loads. The design, does, however, result in a system which contains an additional component, the snubber, which represents an added cost, an additional maintenance item, added weight, and could adversely effect reliability.
The principal object of the present invention is to eliminate the snubber of the earlier flexrotor design and to provide a pitch control system for the flexrotor which not only provides the required collective pitch variation for the tail rotor blades but also serves all functions previously performed by the snubber.
It is an object of this invention to teach a helicopter of the flexrotor type in which the pitch control system not only produces the required collective pitch variation in the tail rotor blades but also supports the blade torque tube with respect to the blade torsionally flexible structural spar.
It is a further object of this invention to teach such a tail rotor and its control system wherein the blade is caused to change pitch and the torque tube is positioned relative to the flexible spar by means of a selectively rotatable wheel member spaced from the torque tube inner end and connected thereto by push-pull rods, which not only serve the torque tube positioning function, but as the control wheel member is caused to rotate, one of the push-pull control rods pulls upon the torque tube inner end while the other push-pull rod pushes upon the torque tube inner end to cause the blade to change pitch.
It is a further object to teach such a control system for a helicopter tail rotor flexrotor which has the advantage of push-pull rod redundancy in that, if one of the two push-pull rods fails, the other could be used to control blade pitch with the torque tube bottomed out against flexbeam spar, as an emergency measure.
It is a further object to teach such a helicopter tail flexrotor in which the blades are caused to change pitch by a mechanism which rotates, rather than translates relative to the tail rotor, thereby eliminating the drag created by the earlier translating tail rotor pitch controls. These prior art translating tail rotor pitch controls are typically of the type shown in U.S. Noehren et al Pat. No. 4,008,980. Other controls which translate for pitch variation are shown in Seibel U.S. Pat. No. 2,575,533 and U.S. Pat. No. 3,957,227. While U.S. Elmer Pat. No. 3,218,808 uses a rotary motion in the fan art, it should be noted that this is a rigid fan blade and utilizes but a single pitch rod. The mechanism is a simple centrifugal pitch control which uses a hinged mass operating against a spring to vary the fan blade pitch as a function of rpm.
Other objects and advantages of the present invention may be seen by referring to the following description and claims, read in conjunction with the accompanying drawings.